Racing boat a stimulus for the Dark Blues? (9)

I believe the answer is:

outrigger

I'm a little stuck... Click here to teach me more about this clue!

'racing boat' is the definition.
'outrigger' can be an answer for 'boat' (I've seen this before). I'm not sure about the 'racing' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for outrigger that I've seen before include "Git, our Reg (anag.); boat" , "Ship's stabilising float" , "Type of boat with external supports" , "Craft" , "Boat stabiliser" .)

I've seen this clue in The Independent.

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